- Store ReAct steps in chat message history as agent_steps
- Save agent_steps to the assistant instead of user message
- Remove unnecessary code and tests
- Add spec

Changelog: fixed
EE: true

# Conflicts:
#	ee/lib/gitlab/llm/chat_storage.rb
#	ee/spec/lib/gitlab/llm/chain/utils/chat_conversation_spec.rb
#	ee/spec/lib/gitlab/llm/chat_storage_spec.rb

# Conflicts:
#	ee/lib/gitlab/llm/chat_storage/postgresql.rb